1. Field of the Invention
The present invention relates to an optical gate device, a manufacturing method for the device, and a system including the device.
2. Description of the Related Art
A Mach-Zehnder interferometer (MZI) type optical gate is known as a conventional optical gate device. This optical gate is configured by integrating a Mach-Zehnder interferometer including first and second nonlinear optical media each for providing a phase shift on an optical waveguide substrate, for example. Probe light as continuous wave (CW) light is equally divided into two components, which are in turn supplied to the first and second nonlinear optical media. The optical path length of the interferometer is set so that output light is not obtained by interference of the two components of the probe light.
An optical signal is further supplied to one of the first and second nonlinear optical media. By properly setting the powers of the optical signal and the probe light, a converted optical signal synchronous with the optical signal is switched out from the optical gate. The converted optical signal has the same wavelength as that of the probe light.
It has been proposed to use a semiconductor optical amplifier (SOA) as each of the first and second nonlinear optical media. For example, an InGaAs-SOA having two (both) end faces treated with antireflection coatings is used as each nonlinear optical medium in a 1.5 xcexcm band, and these nonlinear optical media are integrated on an InP/GaInAsP substrate to fabricate an optical gate.
A nonlinear optical loop mirror (NOLM) is known as another conventional optical gate device. The NOLM includes a first optical coupler including first and second optical paths directionally coupled to each other, a loop optical path for connecting the first and second optical paths, and a second optical coupler including a third optical path directionally coupled to the loop optical path.
By forming a part or the whole of the loop optical path from a nonlinear optical medium and supplying probe light and an optical signal respectively to the first optical path and the third optical path, a converted optical signal is output from the second optical path.
An optical fiber is generally used as the nonlinear optical medium in the NOLM. In particular, a NOLM using a SOA as the nonlinear optical medium is referred to as an SLALOM (Semiconductor Laser Amplifier in a Loop Mirror).
The MZI type optical gate is excellent in size reduction and integration, but its manufacturing technique has not yet been established.
The optical gate having an SOA as the nonlinear optical medium has a problem that amplified spontaneous emission (ASE) noise added by the SOA has an adverse effect on fundamental characteristics including a signal-to-noise ratio (SNR).
On the other hand, the NOLM requires a long fiber to obtain a required nonlinear optical effect. Accordingly, there arises a signal rate limit due to chromatic dispersion, and it is difficult to cope with polarization dependence of an input optical signal and polarization fluctuations in the loop optical path.
It is therefore an object of the present invention to provide an optical gate device allowing the use of a relatively short optical fiber as the nonlinear optical medium, and to also provide a manufacturing method for the device and a system including the device.
In accordance with an aspect of the present invention, there is provided a device including first and second optical couplers and a loop optical path. The first optical coupler includes first and second optical paths directionally coupled to each other. The loop optical path includes an optical fiber as a nonlinear optical medium, and connects the first and second optical paths. The second optical coupler includes a third optical path directionally coupled to the loop optical path. The optical fiber has an enough large nonlinear coefficient. The wording of xe2x80x9cenough largexe2x80x9d means that the nonlinear coefficient is large enough to reduce the length of the optical fiber to such an extent that the optical fiber has a polarization maintaining ability.
In the present invention, by using such an optical fiber having an enough large nonlinear coefficient, a relatively short optical fiber can be used as the nonlinear optical medium. Accordingly, it is possible to provide an optical gate device which can suppress a signal rate limit due to chromatic dispersion and can easily cope with polarization dependence of an input optical signal and polarization fluctuations in the loop optical path.
In accordance with another aspect of the present invention, there are provided first to third manufacturing methods for a device having a first optical coupler including first and second optical paths directionally coupled to each other, a nonlinear optical medium for forming a loop optical path connecting the first and second optical paths, and a second optical coupler including a third optical path directionally coupled to the loop optical path.
The first manufacturing method comprises the steps of (a) cutting an optical fiber into a plurality of sections, and (b) arranging the plurality of sections and joining them together so that a conversion band by a third-order nonlinear effect using the nonlinear optical medium becomes a maximum, thereby obtaining the nonlinear optical medium.
The second manufacturing method comprises the steps of (a) cutting an optical fiber into a plurality of sections, (b) measuring the dispersions of the plurality of sections, and (c) selecting any of the sections having dispersions small enough to obtain a required conversion band by a third-order nonlinear effect using the nonlinear optical medium and joining the selected sections, thereby obtaining the nonlinear optical medium.
The third manufacturing method comprises the steps of (a) measuring a deviation in zero-dispersion wavelength of an optical fiber, (b) cutting the optical fiber into a plurality of sections when the deviation exceeds a predetermined range, and making the deviation in zero-dispersion wavelength of each of the sections fall within the predetermined range, and (c) selecting the optical fiber or the sections having substantially the same zero-dispersion wavelength and joining the selected sections, thereby obtaining the nonlinear optical medium.
In accordance with a further aspect of the present invention, there is provided a system comprising an optical gate device, a probe light source, and first and second optical fiber transmission lines. The optical gate device comprises a first optical coupler including first and second optical paths directionally coupled to each other, a nonlinear optical medium for forming a loop optical path connecting the first and second optical paths, and a second optical coupler including a third optical path directionally coupled to the loop optical path. The probe light source is connected to the first optical path to supply probe light to the first optical path. The first optical fiber transmission line is connected to the third optical path to supply an optical signal to the third optical path. The second optical fiber transmission line is connected to the second optical path to transmit a converted optical signal output from the second optical path.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.